Carbon nanofibers and procedure for obtaining said nanofibers

ABSTRACT

The object of the present invention is carbon nanofibers mainly characterized by their high specific volume of mesopores, their high gas adsorption capacity and presenting a graphitic hollow structure. A second object of this invention is a procedure for obtaining such carbon nanofibers, which makes use of a metallic nickel catalyst and specific process furnace parameters that combined with the chemical composition of the furnace atmosphere and the fluidodynamic conditions of the gas stream inside the furnace, result in a faster growth of the carbon nanofibers and also in a higher quality of the carbon nanofibers obtained.

OBJECT OF THE INVENTION

The object of the present invention are carbon nanofibers that presentimproved physical properties that allow their use in a wide range ofapplications as well as in fields in which the use of carbon nanofibersobtained by other procedures is less effective.

These carbon nanofibers are defined mainly by macroscopic propertiesthat characterize their performance. The most relevant property is thespecific volume of mesopores, which is related to the structuralconfiguration determined by hollow graphitic filament with a large innerdiameter and by high gas adsorption capacity.

Other object of this invention is a procedure for obtaining said carbonnanofibers developed in order to make possible the mass production forindustrial applications and with properties such as a high specificsurface area and a high graphitization degree, obtained withoutrequiring any carbon nanofiber post-treatment. These improved propertiesprove the high quality of the carbon nanofibers so obtained.

The procedure for obtaining the carbon nanofibers is characterized bythe use of a specific metallic catalyst and production parameters thatallow establishing binary phase conditions in said catalyst which,combined with the chemical composition of the furnace atmosphere and thefluidodynamic conditions verified inside the production furnace, providea growth of the carbon nanofibers faster than the corresponding to thestandard production processes known for carbon nanofibers, resulting ina higher quality of the carbon nanofibers.

This higher quality of the carbon nanofibers this way obtained isreflected on their surface, where the accumulation of amorphous carbonof pyrolytic origin is negligible, in contrast with other carbonnanofibers of the state of the art. Consequently, the specific surfacearea is greater, the graphitization degree is higher and the specificvolume of mesopores is higher too. It is therefore observed that theirgas adsorption capacity due to the mesopores volume is considerablyhigher than that provided by other carbon nanofibers.

BACKGROUND OF THE INVENTION

Carbon nanofibers are graphitic filament structures formed by theirgrowth from the catalytic decomposition of a hydrocarbon in gaseousphase. This growth is determined by the kinetic and thermodynamicreaction conditions, the composition of the feedstock gas under hightemperature conditions, and the nature of the metallic catalystemployed.

The various manufacturing methods for carbon nanofibers can beclassified into two main groups, depending on whether the catalyst usedis on a fixed substrate or whether it is a floating catalyst. They arealso determined by the different reaction conditions and the compositionof the working atmosphere, establishing different ranges in theparameters associated to each one.

The production process used to manufacture the carbon nanofibers iscrucial from an industrial point of view, since the different conditionsvalid for obtaining carbon nanofibers in the laboratory may not befeasible from an industrial standpoint due to the limited manufacturingcapacity.

In the case of the floating catalyst technique, the reaction takes placein a specific volume without the metallic catalyst particle beingdeposited on any substrate surface, since it is introduced in thereactor in a continuous manner suspended in the reacting gas flow. Theadvantage of this technique is that there is no need to later take apartthe nanofibers produced from the substrate. In this way, as the supplyof reagents to the process furnace and the collection of the productobtained are continuous, the process for producing carbon nanofibersusing the floating catalyst technique is directly applicable inindustry, unlike carbon nanofiber production processes based ontechniques in which the catalyst particles are deposited on a substratethat is subsequently introduced into the process furnace to activate theformation of the carbon nanofibers and eventually removed to collectthese carbon nanofibers, separating them from the substrate on whichthey have been grown.

The carbon nanofibers of the present invention are formed from themetallic catalyst particles suspended in the process furnace gas flow,forming nanometric graphitic fibrillar structures.

The nanofiber graphitic filaments continue growing until the catalystparticles are poisoned or over-saturated with carbon. After thegraphitic filament has been grown, a filament thickening process wouldtake place that involves the deposition of pyrolytic carbon on thecarbon nanofiber surface, which presents lower structural order than thecatalytic graphitic carbon.

Hereinafter said pyrolytic carbon will be identified as amorphouscarbon. Amorphous carbon has negative effects on the surface activity ofthe nanofiber, as will be commented further below, and therefore alsohas negative effects on its possible applications.

There are studies, such as those by Oberlin [Oberlin A. et al., Journalof Crystal Growth 32, 335 (1976)] that analyze the growth of carbonfilaments on metallic catalyst particles by transmission electronmicroscopy techniques.

Based on these studies, Oberlin proposed a growth model for carbonnanofibers or nanofilaments based on diffusion of carbon about thesurface of the catalyst particles until the surface of these particlesis saturated or poisoned by excess carbon.

Oberlin also explained that the deposition of amorphous carbon bythermal pyrolytic decomposition is the process responsible for thethickening of graphitic filaments previously grown from the metalliccatalyst, and that said pyrolytic process takes place whenever thetemperature of the process furnace is high enough and the residence timeof the carbon nanofibers in the process furnace is long enough.Therefore, after the catalytic carbon filament growth process hasconcluded due to the poisoning or the carbon saturation of the catalystparticle, the filament continues to be thickened if it remains exposedto the pyrolysis conditions for an extended period.

The carbon nanofiber thickening process due to the deposition ofamorphous carbon of pyrolytic origin is very difficult to avoid, due tothe fact that the deposition of pyrolytic amorphous carbon on the carbonnanofiber surface takes place very quickly at production conditions on afloating catalyst system. Thus, only in the case of very low residencetime of the gaseous furnace atmosphere, which transports along thefurnace the catalyst particles and the carbon nanofibers produced in theprocess, is possible to avoid the deposition of pyrolytic amorphouscarbon on the carbon nanofibers surface, thereby avoiding the loss ofquality and properties of the carbon nanofibers resulting in lowergraphitization degree, lower specific surface area value and lowerspecific volume of mesopores.

Considering the structure of the carbon nanofiber graphitic filament ofcatalytic origin, several configuration models have been identified todescribe its graphitic structure based on the different possible growthforms depending on the metallic catalyst particles and the reactionconditions. In some cases the carbon nanofibers are solid and areconfigured by superimposed graphitic stacked flat plates (known as aplatelet structure) and other times their structures are more complexpresenting the well known fishbone structure. In the case of not stackedgraphitic planes, it is possible to differentiate either a hollow orsolid structure composed by the planes in the form of superimposedribbons along the axis of the carbon nanofibers (known as a ribbonstructure). Finally there is other possible structure in the form ofstacked truncated cones (stacked cup structure).

The structure of carbon nanofibers is often modelled by one of theseconfigurations, although in most cases it is difficult to preciselydetermine the actual structure due to the limitations of the inspectionand analysis instruments. This is why we refer to “modelling”, as it isunderstood that there is a reasonable fit with one of the aforementionedstructures.

FIGS. 3 and 4 are transmission electron microscopy images of a carbonnanofiber according to an embodiment of the invention showing the end ofa carbon nanofiber wherein its spiral ribbon is partially unwoundedshowing a periodically twisted structure. These figures will be used inthe detailed description of the invention.

In our case it is not only a model, as the experiments conducted confirmthe structure of the nanofiber, as the pictures of FIGS. 3 and 4 clearlyshow.

There is no doubt that the structure of the carbon nanofibers determinestheir physical properties at a macroscopic level when used in industrialapplications.

For example, structures consisting of disjointed planes would result ina lower electrical conductivity of the filament than in structures whichgraphitic planes are continuous along the fiber axis providing a highconductivity.

A similar argument applies to the specific surface area of the carbonnanofibers. The free edges of the basal graphitic planes formed duringthe filament growth are important in all of these carbon nanofiberproperties. These free edges increase the specific surface area of thecarbon nanofibers and consequently they favour gas adsorption andinteraction with other substances to form chemical bonding.

If the external structure of the carbon nanofiber, and more specificallythe basal plane free edges, are covered by pyrolytic amorphous carbonthen this forms a passivating barrier that hinders the chemical activityof the carbon nanofibers, reducing their capacity to interact with othersubstances or molecules and reducing the final specific surface area. Inthis case the fiber will have a poorer quality and fewer applications.

Similarly, a greater presence of pyrolytic amorphous carbon in thecarbon nanofibers implies a reduced graphitization degree, which isclosely related to their physical properties such as the thermal andelectric conductivities; in short, it affects the final quality of thecarbon nanofibers and their possible applications.

U.S. Pat. No. 5,024,818 is known, which describes a method for producingcarbon nanofibers from carbonaceous compounds. In this patent it isspecified that Fe is used as a catalyst. The furnace described in thispatent is a floating catalysis process furnace.

Based on the data supplied in the patent, it can be inferred that thefurnace operates from 1100° C. to 1150° C. (around 1140° C.), and thatit uses a mixture of Fe compounds and S compounds with a molar ratio of1/1.

Similarly, it can also be inferred that the residence time of thegaseous reagents under the process conditions inside the furnacedescribed in this patent is about 30 s, with a travel velocity of thesegases from 0.011 m/s to 0.033 m/s.

The gas stream velocity inside the furnace is important from aproduction standpoint. This gas stream velocity inside the furnace isdirectly related to the ratio [L₀]/[t₀], where [L₀] is thecharacteristic length of the furnace and [t₀] is the characteristicresidence time of the gaseous mixture in the furnace.

The production capacity is determined by the [L0] of the processfurnace, by the gas stream velocity and by the residence time [t₀]needed. The larger the [L₀] of the furnace, the higher the gas streamvelocity and the shorter the residence time [t₀] are the greater theproduction capacity of the furnace process is.

The minimum residence time [t₀] needed is determined by the timerequired for nucleation and growth of the carbon nanofibers. Thecharacteristic furnace length [L₀] is mainly constrained by constructivelimitations derived from limited features of the materials currentlyavailable for manufacturing such kind of furnaces.

The need for a high [L₀] would lead to a greater size of the furnacethat can make its construction unfeasible. Current techniques of makingthis hind of furnaces do not allow exceeding certain size of thefurnace.

These are the main physical limitations, so that it is of interest toact on the residence time [t0] required for the carbon nanofibernucleation and growth allowing this way to increase the averagecirculation velocity inside the furnace and therefore increasing theproduction capacity of the furnace accordingly.

However the circulation velocity cannot be increased arbitrarily. Theaforementioned U.S. Pat. No. 5,024,818 sets a limit for the velocity ofthe gas stream carrying the Fe catalyst particles to avoid turbulentregimes, as otherwise the fluctuations resulting from the vorticity willprevent a stable growth. This patent establishes for one of thepreferred embodiments a circulation velocity of 0.033 m/s.

The object of this patent is to determine the conditions of the reactionprocess in which the average stream velocity exceeds even by severalorders of magnitude those velocities used in U.S. Pat. No. 5,024,818patent, obtaining accordingly a much higher production capacity andtherefore significantly increasing its industrial applicability.

Also the object of this patent is the carbon nanofibers obtained withimproved properties, particularly their specific surface area, theirgraphitization degree and their specific volume of mesopores, therebyimproving their gas adsorption capacity, their physical properties andfinally their overall quality, allowing a wide industrial applicability.

DESCRIPTION OF THE INVENTION

The carbon nanofibers object of the invention presents a hollowstructure with a circular cross-section. It is formed by graphiticplanes arranged in the form of spirally superimposed ribbons along theaxis of the carbon nanofibers thereby ensuring its continuity, so thatamong its noteworthy properties are its high electric and thermalconductivities. This structure will be shown in the drawings and theelectron microscopy pictures of the detailed description of theinvention.

The carbon nanofiber surface presents two families of free edges able toform bonds with other substances: those facing the outside of thenanofiber and those facing the inner cylindrical channel of the carbonnanofiber.

Although most the carbon nanofibers obtained by the manufacturingprocess of the invention present a structure corresponding to a spirallysuperimposed ribbon model, it is noted that other kind of structures arealso present as a smaller part of the final product, such as plateletcarbon nanofibers or carbon nanotubes.

The presence of pyrolytic amorphous carbon on the outer surface of thecarbon nanofibers covering at least part of the aforementioned graphiticribbons free edges, thereinafter the free plane edges, reduces thecapacity of the carbon nanofibers to interact with other chemicalcompounds. Therefore, one of the advantages of the carbon nanofibers ofthe invention is a greater specific surface area due to the absence ofpyrolytic amorphous carbon using the procedure of the invention.

The most important of scattering techniques has been used as a dominanttool to study the defect structure of all types of carbon fibers andfilaments. Many carbons have disorder between that of the two extremetheoretical situations, single crystal graphite and turbostratic carbon.This disorder is usually known as turbostratic disorder and it has aninverse linear relationship with the graphitization degree.

Several attempts have been made to quantify the graphitization degree.The simplest approach is to use an empirical graphitization index Gp,which depends linearly on the interplanar separation d002, which isobtained from the (002) reflection of the X-ray diffraction test [Maire,J. and Mering, J., 1958, Proc. First Conference of the Society ofChemical Ind. Conf. on Carbon and Graphite (London), 204] using therelation between d₀₀₂ and G_(p):

G _(p)=[(0,344-d ₀₀₂)/(0,344-0,3354)]·100

Regarding the free plane edges facing the inner channel of the carbonnanofiber, these can have a greater or lesser capacity to adsorb gaseoussubstances.

The first object of the invention is a carbon nanofiber with gasadsorption capacity substantially higher than that observed in the priorart.

The free plane edges facing the carbon nanofiber inner channelcontribute to the gas adsorption capacity. The pyrolytic amorphouscarbon does not contribute to the formation of graphitic planes.Therefore a greater proportion of said amorphous carbon reduces thenumber of active plane free edges formed per unit mass and consequentlythe gas adsorption capacity of the product obtained.

The carbon nanofibers gas adsorption is mainly determined by thecapacity to retain gaseous atoms, ions or molecules depending mainly onthe chemical affinity of the graphitic plane free edges to them, both onthe inner and outer surfaces of the carbon nanofiber. This gasadsorption is enhanced in the inner surface of the carbon nanofiber dueto the capillarity effects resulting from the dimensions of the innerchannel diameter. For this reason, the inner plane free edges contributemore to the gas adsorption capacity of the carbon nanofibers than theouter ones.

The inner diameter of the hollow channel of the carbon nanofibers objectof the invention is in the range from 2 nm to 50 nm. This rangecorresponds to a specific kind of pores referred as mesopores by theIUPAC (The International Union of Pure and Applied Chemistry). Thus, itcan be said that the resulting gas adsorption capacity of the carbonnanofibers is mainly due to the presence of mesopores in theirstructure.

This fact is corroborated by the N₂ adsorption-desorption test at 77K ofthe carbon nanofibers without any activation treatment. Thecorresponding N₂ adsorption-desorption isotherms obtained show anhysteresys cycle curve which lower limit corresponds to relativepressure values P/P₀ (where P₀ is the N₂ saturation pressure) at about0.4 and always under 0.65, corresponding the P/P₀ range covered by thishysteresys cycle to the effect of the presence of mesopores. Thecorresponding specific mesopore volume of the carbon nanofibers issignificantly higher than that found in the carbon nanofibers known inthe prior art.

The gas adsorption capacity of the carbon nanofiber surface depends onthe presence of mesopores and the total specific volume of mesopores.

A second aspect of the present invention consists of a procedure forobtaining carbon nanofibers from a enriched carbon gas stream and afloating metallic catalyst, with a high production capacity due to ahigh circulation velocity of the gas stream in the carbon nanofiberproduction furnace, as well as improved properties of the carbonnanofibers obtained with this process.

The manufacturing procedure involves the use of a process furnace forgenerating carbon nanofibers from a gas mixture stream that circulatesthrough it, with floating metallic catalyst particles, causing thenucleation and subsequent growth of carbon nanofibers.

The main factor allowing a high nucleation rate and fast growth of thecarbon nanofibers is the use of a suitable combination of metalliccatalytic particles and a gas stream with specific properties.

This invention uses nickel metal particles which, at the process furnacetemperature, combined with the sulfur present in the furnace gaseousatmosphere to attain at these conditions a binary phase in the catalystparticle, a solid Ni phase and a liquid NiS phase.

In these conditions at the process furnace temperature the catalyticparticle undergoes a partial fusion so that the particle made up of twophases, differentiating in the catalytic particle a Ni core that remainssolid throughout the entire process and a NiS part due to the presenceof sulfur in the gases involved in the process that at the processtemperature will fuse due to the melting point of NiS is lower that theworking temperature, and thus can flow around the solid Ni core.

As alternative it is also possible to supply to the process furnacesulfur used to generate the catalytic particle as a component of achemical compound added to the gas process stream.

The liquid NiS phase allows the deformation of the catalytic particlewhen it is carried by the gas stream, adopting an elongated or teardropshape in the reaction conditions allowing a faster carbon nanofibernucleation and growth.

The addition of the catalytic particles to the gas process stream in thefurnace when the gas stream velocity is high enough can generate itslengthening due to the deformation of the NiS liquid phase of each ofthem attaining optimal carbon nanofiber growing conditions.

There are two main reasons because the presence of a combination of Niand S is necessary: firstly, because nickel has been proved to be moreactive than iron in carbon nanofibers nucleation and growth; secondly,because the NiS physical properties allow it to melt at the processtemperature.

The combined use of a mixture of Ni and NiS as a catalyst is the mainreason for the enhanced production of carbon nanofibers at a higherproductivity and with better carbon nanofiber quality, this is, with ahigh specific surface area and graphitization degree.

This higher carbon nanofiber nucleation and growth rates allow the useof higher gas stream process velocities that can even be in theturbulent regime without harming the final quality of the carbonnanofibers so obtained.

The formation of amorphous carbon of pyrolytic origin is favoured bylonger residence times in the process furnace at the workingtemperature. Part of the improved quality of the carbon nanofibersobtained by the procedure of the invention is due to the shorterresidence time needed for the carbon nanofiber nucleation and growth andconsequently the formation and deposition of amorphous carbon ofpyrolytic origin on the outer surface of the nanofiber is considerablyreduced.

It can be established that the essential elements of the second aspectof the invention a procedure for obtaining carbon nanofibers in afurnace from the catalytic decomposition of a hydrocarbon in vapourphase using a floating catalyst, by the growth of the graphiticfilament, wherein:

The catalytic particle comprises two phases, a first solid phase ofmetallic nickel and a second phase of a nickel sulfide compound inliquid state during the growth of the graphitic filament; and

The catalytic particle is carried by the gas stream inside the processfurnace.

DESCRIPTION OF THE FIGURES

The present descriptive report is completed by a set of figures thatillustrate a preferred example and in no way limit the invention.

FIG. 1 shows the model describing the structure of a carbon nanofiber invarious growth stages. There is a first stage when the catalyticparticle is composed of two phases, a solid nickel phase and a moltennickel sulfide phase. A second stage is shown where the formation of thespirally superimposed ribbons along the axis of the carbon nanofibersdue to the desorption of carbon supplied by the gas stream isrepresented by the thick arrows. The third stage shown corresponds tothe further growing of the graphitic filament.

FIG. 2 shows a graph comparing the two N₂ adsorption-desorptionisotherms corresponding to two samples of carbon nanofibers, a firstsample corresponding to the carbon nanofibers object of this inventionand a second sample corresponding to the prior art carbon nanofiberscited in the present description. The lower end of the hysteresis cyclewithin the isotherm corresponding to the carbon nanofibers of theinvention is labelled as (I); the lower end of the hysteresis curvecycle within the isotherm corresponding to the cited prior art carbonnanofibers is labelled as (II).

FIGS. 3 and 4 are transmission electron microscopy images of a carbonnanofiber according to an embodiment of the invention showing the end ofa carbon nanofiber wherein the spiral ribbon is partially unwoundedshowing a periodically twisted structure.

DETAILED DESCRIPTION OF THE INVENTION

To characterize the carbon nanofibers according to the first aspect ofthe invention, laboratory experiments have been carried out that showtheir main properties. The carbon nanofibers obtained by the procedureof the same invention present improved physical properties that allowtheir use in a wide range of applications as well as in fields in whichthe use of carbon nanofibers obtained by known procedures of the priorart is less effective.

Due to the wide range of values of the characteristics of the carbonnanofibers obtained from the floating catalyst process, the propertiesof the carbon nanofibers obtained must be statistically studied. Thecharacteristics of any single graphitic filament do not represent theglobal properties of the material. Therefore, to evaluate the carbonnanofibers properties it is necessary the use of characterizationtechniques applied to samples large enough to be representative of thematerial. For this reason, the characterization of the carbon nanofibersof the invention is mainly based on the measure of their specificproperties as the specific surface area, the specific mesopore volumeand the graphitization degree.

In the case of the specific mesopores volume, it is determined from theisotherms resulting of the N₂ adsorption-desorption experiment.

In this experiment, due to the differences between the N₂ adsorption anddesorption processes resulting from the presence of mesopores in thecarbon nanofibers, two different isotherm curves are recorded, a firstisotherm corresponding to the N₂ adsorption process and a second onecorresponding to the N₂ desorption process, both determining anhysteresys cycle, being the surface enclosed in the hysteresys curvedirectly proportional to the specific volume of mesopores.

The measurement method subjects a sufficiently representative amount ofcarbon nanofibers to a constant temperature in a controlled atmospherecomposed of the gas to be adsorbed and desorbed, which in this case area temperature of 77K and pure N₂ the gas to be used.

This pure N₂ atmosphere is subjected to a rising-decreasing pressurecycle at isothermal conditions, consequently inducing a N₂adsorption-desorption process in the carbon nanofibers sample. The testconditions to carry out the experiment are defined by standard DIN66131:1993 which is considered for both, the rising and the decreasingpressure cycles. The experiment has been carried out according to theIUPAC (Pure & Appl. Chem., Vol 57, No, 4, pp 603-619, 1985)recommendations in order to quantify the porosity and the specificsurface area. In particular, standard DIN 66131:1993 has been used toevaluate the specific surface area.

The N₂ adsorption-desorption isotherms form a closed hysteresys cyclewhich enclosed area is proportional to the specific volume of mesopores.These curves are represented in FIG. 2 obtained from two samples, one ofthem corresponding to the sample of carbon nanofibers of the inventionand the other sample corresponding to the carbon nanofibers of the priorart.

It is highly relevant to determine the point at which the relativepressure P/P₀ of N₂ adsorption and desorption isotherms begin todiverge. The lower this point, the larger the area enclosed by thehysteresys cycle and consequently the greater the adsorption capacity.This way it is possible to determine values for the P/P₀ lowerhysteresys cycle point under 0.65 characterizing the carbon nanofibersof the present invention. Moreover, the carbon nanofibers obtainedaccording to the procedure of this invention can even reach values forthe P/P₀ lower hysteresys cycle point of about 0.4.

Applying this experimental technique to both, the carbon nanofibers ofthe present invention and to the carbon nanofibers obtained according tothe procedure described in U.S. Pat. No. 5,024,818, the followingspecific mesopore volume values are obtained:

0.18 cm³/g for the carbon nanofibers obtained according to the presentinvention:

0.04 cm³/g the carbon nanofibers obtained according to the proceduredescribed in U.S. Pat. No. 5,846,509:

Therefore, the difference of the specific volume of mesopores of the twocarbon nanofibers can be even as large as 450%.

The lowest value found for the specific volume of mesopores for thecarbon nanofibers object of the invention is 0.08 cm³/g. Allmeasurements correspond to 77K isothermal conditions and without anysurface activation treatment of the carbon nanofibers surface that maymodify the surface activity of the graphitic filament.

FIG. 2 shows the N₂ adsorption-desorption isotherms. The largesthysteresys cycle corresponds to the carbon nanofibers of the presentinvention and the smaller one corresponds to those carbon nanofibersobtained by the procedure described in U.S. Pat. No. 5,846,509.

The specific surface area and graphitization degree have been alsomeasured obtaining higher values of both properties for the carbonnanofibers of the present invention than for the carbon nanofibersproduced by the procedure described in U.S. Pat. No. 5,846,509 of theprior art.

Regarding the values measured for the carbon nanofibers of the presentinvention, the specific surface area was found to be always higher than100 m²/g, whereas the carbon nanofibers of the prior art does not exceeda specific surface area of 50 m²/g. More specifically the carbonnanofibers of the present invention present specific surface area valuesfrom 130 to 200 m²/g.

Regarding the graphitization degree, quantified by the graphitizationindex, the values obtained for the carbon nanofibers of the presentinvention was found to be above 40%, whereas the carbon nanofibersdescribed in U.S. Pat. No. 5,846,509 present values between 30% to 40%.More specifically the carbon nanofibers of the present invention presentgraphitization index values from 50% to 75%.

The carbon nanofibers structure of the present invention is a graphiticfilament being essentially cylindrical and hollow, identifying bytransmission electron microscopy that its longitudinal cross sectionshows graphitic planes oblique to the longitudinal axis of the filament.

These oblique planes observed in the cross section correspond to thebasal graphitic planes that constitute the helical superimposed ribbonstructure of the carbon nanofibers in which said planes are thereforeconnected to each other along the axis of the carbon filament as shownin FIGS. 3 and 4.

Since the negligible formation of amorphous pyrolytic carbon cannotcover the graphitic carbon filament in its greater part, the edges ofthe basal graphitic planes are not covered by amorphous pyrolytic carbonand therefore the resulting surface of the carbon nanofibers of thepresent invention keeps a high chemical activity. That is why the carbonnanofibers of the present invention present a specific surface area andgraphitization degree much higher than those observed for carbonnanofibers of the prior art.

Other factor that can contribute to a higher specific volume ofmesopores of the carbon nanofibers is the falling off of the catalyticparticles from the carbon nanofibers, leaving the end of the nanofiberopen allowing the access of gaseous atoms, molecules or ions inside thehollow core of the graphitic filament. However, carbon nanofibers withthe catalytic particles attached to their ends are also considered to bepart of the invention.

The process for obtaining the carbon nanofibers of the invention uses asulfur supply in the gas stream introduced into the furnace, being thefurnace temperature between 900° C. and 1250° C., being the preferredtemperature range from 1100° C. to 1200° C.

The sulfur-nickel molar ratio used must be in the range 0.5 to 5, beingpreferred the range from 1.2 to 3. In these ranges and at the specifiedfurnace temperature the catalytic particles present a binary phasecomposed of a solid metallic nickel phase and a molten nickel sulfidephase.

To prevent the formation of amorphous carbon of pyrolytic origin thatcould deteriorate the properties of the carbon nanofibers obtained bythe process of the present invention the gas stream velocity inside thefurnace must be between 0.1 m/s and 12 m/s, and preferably between 0.2m/s to 1.5 m/s.

These circulation velocity ranges ensure a residence time of the carbonnanofibers produced in the furnace that is in the range from 1 s to 15s, and preferably from 2 to 8 s.

The FIGS. 3 and 4 correspond to different transmission electronmicrographs of the end of different carbon nanofibers obtained accordingto the process of the present invention where a partially unwounddetached end of the helical superimposed graphitic ribbon planes of thecarbon nanofiber is shown.

The end and the detached part of the carbon nanofiber are indicated inthe figures. The micrographs show the helical superimposed graphiticribbon planes of the hollow structure of the carbon nanofiber apparentlysimilar to but different from a cup-stacked graphitic structure.

According to this micrographs, the ribbon is cyclically stackedfollowing a continuous helical generatrix being constituted by one ormore than one graphitic planes. In some views looking at the point wherethe detached part of the ribbon narrows it is also possible to observethe presence of one or several graphitic planes.

1. Procedure for obtaining carbon nanofibers in a furnace from thecatalytic decomposition of a hydrocarbon in vapour phase using afloating catalyst, by the growth of the graphitic filament, wherein: Thecatalytic particle comprises two phases, a first solid phase of metallicnickel and a second phase of a nickel sulphide compound in liquid stateduring the growth of the graphitic filament; and The catalytic particleis carried by the gas stream inside the process furnace.
 2. Procedurefor obtaining carbon nanofibers according to claim 1 characterized inthat the sulfur contained in the nickel sulfide of the catalyticparticle is supplied to the furnace gas stream atmosphere.
 3. Procedurefor obtaining carbon nanofibers according to claim 1 characterized inthat the catalytic particle is generated inside the furnace from achemical compound including sulfur in its chemical composition. 4.Procedure for obtaining carbon nanofibers according to claim 1characterized in that the process temperature is in the range from 900°C. to 1250° C.
 5. Procedure for obtaining carbon nanofibers according toclaim 4 characterized in that the process temperature is in the rangefrom 1100° C. to 1200° C.
 6. Procedure for obtaining carbon nanofibersaccording to claim 1 characterized in that the sulfur-nickel molar ratiois in the range from 0.5 to
 5. 7. Procedure for obtaining carbonnanofibers according to claim 6 characterized in that the sulphur-nickelmolar ratio is in the range from 1.2 to
 3. 8. Procedure for obtainingcarbon nanofibers according to claim 1 characterized in that thecirculation velocity of the gas stream in the furnace is from 0.1 to 12m/s.
 9. Procedure for obtaining carbon nanofibers according to claim 8characterized in that the circulation velocity of the gas stream in thefurnace is from 0.2 to 1.5 m/s.
 10. Procedure for obtaining carbonnanofibers according to claim 1 characterized in that the residence timeof the gas stream in the process furnace is in the range from 1 to 15 s.11. Procedure for obtaining carbon nanofibers according to claim 10characterized in that the residence time of the gas stream in theprocess furnace is in the range from 2 to 8 s.